Diet is believed to play a major role in the development of sporadic colon cancer. The azoxymethane (AOM) model of experimental colon cancer mimics many features of human colon cancer, including tumor promotion by Western style high fat diets. Inflammation is thought to contribute to diet-enhanced tumorigenesis. We recently showed that tumor promotion induced by dietary fat required epidermal growth factor receptor (EGFR) signals and involved increased pro-inflammatory cyclooxygenase-2 (Cox-2). In contrast, we demonstrated that dietary supplementation with a vitamin D analogue F6-D3 inhibited inflammation and tumorigenesis. Vitamin D effects are mediated by the vitamin D receptor (VDR). We showed that the VDR could antagonize signals from key inflammatory and oncogenic transcription factors, including NFkB and b-catenin. In recent preliminary studies, we demonstrated that EGFR signals, which are up-regulated in tumorigenesis, suppressed VDR expression. Furthermore, VDR null mice appear to be more susceptible to tumorigenesis. In Aim 1 using the AOM model we will characterize the relationships between diet and EGFR signals and VDR down- regulation and identify VDR pathways that inhibit diet-promoted inflammation and tumorigenesis. We will utilize AOM-treated VDR wild type and VDR null mutant mice to dissect VDR pathways. We will modulate dietary fat to assess the effects of diet on inflammation and tumorigenesis. We will also include a diet supplemented with F6-D3 alone, or F6-D3 plus EGFR inhibitor Gefitinib to dissect VDR and EGFR signals in diet-promoted inflammation and tumorigenesis. We postulate that dietary fat will increase EGFR signals and decrease VDR expression, whereas F6-D3 will increase VDR levels and reduce inflammatory and oncogenic signals and inhibit tumorigenesis. In Aim 2 we will dissect EGFR pathways in colon cancer cells and colonic fibroblasts that down-regulate VDR and uncover VDR mechanisms that oppose EGFR and pro- inflammatory cytokine signals. Caco-2 colon cancer cells and colonic fibroblasts express functional EGFR and VDR. In preliminary studies, EGF down-regulated the VDR in these cells, whereas EGFR inhibition or active vitamin D up-regulated VDR. Thus, EGFR activation in these transformed and non-transformed colonic cells mimics in vitro the effects of EGFR signals on VDR expression in colonic tumorigenesis in vivo. We will uncover the transcriptional and post-transcriptional mechanisms that mediate this VDR down-regulation by EGFR. Potential mechanisms include inhibited promoter activation, and reduced transcript or protein half-life. Using inducible dominant negative EGFR expressing transfectants to control EGFR, and siRNA and cDNA strategies to modulate VDR levels, we will also dissect EGFR and VDR pathways that control inflammation and oncogenesis. Using pathway-specific proteomic and genomic approaches, we will also search for new EGFR and VDR-regulated cytokine and growth factor signals that control inflammation in vitro and test their relevance in our in vivo models. These studies will identify potential new targets to exploit for chemopreventive strategies. PUBLIC HEALTH RELEVANCE: Colon cancer is the 3rd leading cause of cancer-related deaths in the US. More than half of these malignancies are believed to be preventable by dietary modifications. We have shown that tumor promotion by Western style high fat diets requires epidermal growth factor receptor (EGFR) signals that inhibit anti-inflammatory vitamin D signals. We will uncover EGFR and vitamin D cellular pathways that interact to modulate dietary inflammation and colonic tumor risk.